Note that the black swipe is not caused by Bartender4, if you just use plain Bartender4 it works perfectly (at least for me, perhabs sans the pet bar, not sure if I fixed that yet, should remember tomorrow to check)
Its caused by some cooldown count addons, for example OmniCC causes it, since it overrides the transparency of the swipe.
Technically its a Blizzard bug, but Bartender tries its best to work around it, no good if OmniCC breaks it agian though...
The same may be true for some other action bar addons.